This week on HST

HST Programs: December 2 - December 8, 2013

Selected highlights

GO 12970: Completing the Census of Ultracool Brown Dwarfs in the Solar Neighborhood using HST/WFC3

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying a number of extremely interesting sources, including at least 4 objects that have been confirmed as dwarfs with temperatures lower than 350K. These are among the first examples of Y dwarfs, and all are too faint to be characterised with any degree of certainty using ground-based observations. The current program will use WFC3 G102 grism spectroscopy to verify the nature of a further 20 candidates.

GO 13319: COS Spectroscopy of Pulsating, Metal-Rich, Extremely Low Mass White Dwarfs

The surface-temperature map on a pulsating white dwarf (Figure by Mike Montgomery U. Texas group)

White dwarfs are compact, electron-degenerate remnants that represent the final evolutionary stage for stars less massive than ~7 Msun. White dwarfs emerge from planetary nebulae with extremely high surface temperatures, but with no central energy source, they simply cool like a brick. As they cool, the spectral energy distribution and the spectral characteristics evolve with time. Most white dwarfs have thin hydrogen envelopes, and are therefore have strong Balmer-series absorption lines in the optical at temperatures above ~8,000 degrees (DA white dwarfs), although a sizeable minority have helium envelopes and spectra dominated by helium lnes (DB white dwarfs). In most cases, the mass of the white dwarf scales with the mass of the progenitor, with typical masses around 0.6 MSun for 1-2 solar mass progenitors. A small number of white dwarfs, however, have much lower masases, closer to ~0.2 MSun. These extremely low-mass (ELM) white dwarfs are generally believed to have formed through binary evolution, with the envelope of the progenitor stripped through mass loss and companion accretion during the red giant phase, truncating evolution before the core could complete its growth. The present program aims to obtain COS UV spectra of two ELM white dwarfs, and has two goals in mind: first, some mdoels sugegst that these ELM white dwarfs are extremely metal-rich, and the COS observations will enable direct measurement of the atmospheric metallicity and determination whether these abundances are intrinsic or due to dust accretion; and, second, time-tag measurements with COS can be used to determine whether the degenerates are p-mode pulsators.

GO 13472: The Hubble Constant to 1%? STAGE 4: Calibrating the RR Lyrae PL relation at H-Band using HST and Gaia Parallax Stars

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cephids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. The absoltue calibration of that relationship, however, rests on only 4 nearby RR Lyraes with trigonometric parallax measurements. The present program aims to add to the sample of astrometricall well-observed RR Lyraes by using spatial scanning on WFC3 to determine accurate parallaxes for a sample of Galactic variables lying at distances up to several kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual stars, and an overall distance scale calibration accurate to better than 3%%.

GO 13609: Investigating the Trigger Mechanism for Newly Discovered Main Belt Comet P/2013 P5

Asteroid 596-Scheila, the prototype main-belt coment, imaged by Peter Lake in December, 2010

The term 'comet" is generally associated with low-mass, volatile-rich solar system objects that spend most of their life at very lage distances from the Sun, plunging only rarely into the inner regions where they acquire extended tails due to outgassing. Sometimes those obejcts are captured into short-period, eccentric orbits, leading to rapid depletion of the volatile content in rapidly-successive perihelion passages. However, recent years have seen growing evidence of another class of cometes exist: comets with near-circular orbits that place them between Mars and Jupiter, within the realm of the Main Belt of asteroids. One of the first candidate main belt comets, as these objects have been dubbed, is the asteroid Scheila. Discovered by the Heidelberg astronomer August Kopff in 1906, and named after an English student with whom he was acquainted, this is one of the larger known asteroids, with a diameter estimated as ~110 km. Early December 2010, Steve Larson (of Arizona's Lunar and Planetary Laboratory) noted that Scheila had sprouted a coma halo in observations taken by the Catalina Sky Survey. An examination of archival images revealed no evidence for activity throughout October and November, but a possible onset on December 3rd. The asteroid 1979 OW7/1996 N2 exhibited similar behaviour in 1996 and again in 2002; the initial outburst was ascribed to a collision, but the second event suggests that the activity is intrinsic rather than externally stimulated. More recently, the Pan-STARRS survey has contributed two objects: the asteroid 2006 VW139, imaged during an outburst; and Main Belt Comet 2013-P5. The present HST target-of-opportunity program has been triggered in response to the latter discovery, using high-resolution visual imaging with Wide-Feld Camera 3 to probe the detailed nature of the outburst. The first set of observations were taken on September, revealing significant changes in morphology within a two-week period. The present program builds on those reslts by mapping out a series of ~monthly observations to track the subsequent decline in activity with the aim of better understanding the mechanism that triggered the outburst.